Conventional transportation fuels and many fine chemicals (e.g. monomers, polymers, plasticizers, adhesives, thickeners, aromatic and aliphatic solvents, etc.) are typically derived from non-renewable raw materials such as petroleum. However, there is increasing concern that the use of petroleum as a basic raw material contributes to environmental degradation (e.g., global warming, air and water pollution, etc.) and fosters overdependence on unreliable petroleum supplies from politically unstable parts of the world.
For example, burning a gallon of typical gasoline produces over 19 pounds of carbon dioxide. Because no carbon dioxide is consumed by a refinery in the manufacture of gasoline, the net carbon dioxide produced is always at least as great as the amount of carbon contained in the fuel. In addition, the net carbon dioxide produced by burning a gallon of petroleum-based gasoline can be even higher when the combustion of additional fossil fuels required to power the refinery and the transportation vehicles that bring the fuel to market are considered. In contrast to fossil fuels, when biomass is converted into biofuels, carbon dioxide is removed from the atmosphere. In contrast for a biofuel process, it is possible and probable that the net carbon dioxide of burning a gallon of biofuel is less than the carbon dioxide corresponding to the carbon atoms in the biofuel. Thus, a defining feature of biofuels and biofuel blends is that the net carbon dioxide produced by burning a gallon of biofuel or biofuel blend is less than the net carbon dioxide produced by burning a gallon of gasoline. Thus, biofuels and biomass-derived organic chemical materials provide significant environmental benefits. There is thus a need to identify new renewable sources of raw materials useful for e.g. transportation fuels and as raw materials for the chemical industry, which are scalable and cost-competitive with conventional sources.
Market fluctuations in the 1970s, due to the Arab oil embargo and the Iranian revolution, coupled with the decrease in US oil production led to an increase in crude oil prices and a renewed interest in renewable materials. Today, many interest groups including policy makers, industry planners, aware citizens, and the financial community are interested in replacing or supplementing petroleum-derived fuels with biomass-derived biofuels. One leading motivation for developing biofuels is economic, namely the concern that the consumption rate of crude oil may soon exceed the supply rate, thus leading to significantly increased fuel cost.
Biofuels are renewable transportation fuels which have a long history ranging back to the beginning on the 20th century. As early as 1900, Rudolf Diesel demonstrated an engine running on peanut oil. Soon thereafter, Henry Ford demonstrated his Model T running on ethanol derived from corn. However, petroleum-derived fuels displaced biofuels in the 1930s and 1940s due to increased supply and efficiency at a lower cost.
At present, biofuels tend to be produced using local agricultural resources in many relatively small facilities, and are viewed as providing a stable and secure supply of fuels independent of the geopolitical problems associated with petroleum. At the same time, biofuels can enhance the agricultural sector of national economies. In addition, environmental concerns relating to the possibility of carbon dioxide related climate change is an important social and ethical driving force which is triggering new government regulations and policies such as caps on carbon dioxide emissions from automobiles, taxes on carbon dioxide emissions, and tax incentives for the use of biofuels.
The acceptance of biofuels depends primarily on their economical competitiveness compared to petroleum-derived fuels. As long as biofuels are more expensive than petroleum-derived fuels, the use of biofuels will be limited to specialty applications and niche markets. Today, the primary biofuels are ethanol and biodiesel. Ethanol is typically made by the fermentation of corn in the US and from sugar cane in Brazil. Ethanol from corn or sugar cane is competitive with petroleum-derived gasoline (exclusive of subsidies or tax benefits) when crude oil stays above $50 per barrel and $40 per barrel, respectively. Biodiesel is competitive with petroleum-based diesel when the price of crude oil is $60/barrel or more.
In addition to cost, the acceptance of biofuels is predicated on their performance characteristics, their ability to run in many types of existing equipment, and their ability to meet demanding industry specifications that have evolved over the last century. Fuel ethanol has achieved only limited market penetration in the automotive market in part due to its much lower energy content compared to gasoline, and other properties (such as water absorption) that hinder its adoption as a pure fuel. To date, the maximum percentage of ethanol used in gasoline has been 85% (the E85 grade), and this has found use in only a small fraction of newer, dual-fuel cars where the engines have been redesigned to accommodate the E85 fuel.
Acceptance of biofuels in the diesel industry and aviation industry has lagged even farther behind that of the automotive industry. Methyl trans-esterified fatty acids from seed oils (such as soybean, corn, etc.) have several specific disadvantages compared to petroleum-derived diesel fuels, particularly the fact that insufficient amounts of seed oil are available. Even under the most optimistic scenarios, seed oils could account for no more than 5% of the overall diesel demand. Furthermore, for diesel and aviation engines, the cold flow properties of the long chain fatty esters from seed oils are sufficiently poor so as to cause serious operational problems even when used at levels as low as 5%. Under cold conditions, the precipitation and crystallization of fatty paraffin waxes can cause debilitating flow and filter plugging problems. For aviation engines, the high temperature instability of the esters and olefinic bonds in seed oils is also a potential problem. To use fatty acid esters for jet fuel, the esters must be hydrotreated to remove all oxygen and olefinic bonds. Additionally, jet fuels must contain aromatics in order to meet the stringent energy density and seal swelling demands of jet turbine engines. Accordingly, synthetic jet fuels including hydrotreated fatty acid esters from seed oils, or synthetic fuels produced from coal, must be blended with aromatic compounds derived from fossil fuels to fully meet jet fuel specifications.
Aromatic compounds are conventionally produced from petroleum feedstocks in refineries by reacting mixtures of light hydrocarbons (C1-C6) and naphthas over various catalysts at high heat and pressure. The mixture of light hydrocarbons available to a refinery is diverse, and provides a mixture of aromatic compounds suitable for use in fuel once the carcinogenic benzene is removed. Alternatively, the hydrocarbon feedstocks can be purified into single components to produce a purer aromatic product. For example, aromatization of pure isooctene selectively forms p-xylene over some catalysts. The by-products of these reactions are very light fractions containing hydrogen, methane, ethane, and propane gases which are captured at the refinery for other uses.
Low molecular weight alkanes and alkenes can also be converted into aromatic compounds such as xylene using a variety of alumina and silica based catalysts and reactor configurations. For example, the Cyclar process developed by UOP and BP for converting liquefied petroleum gas into aromatic compounds uses a gallium-doped zeolite (Appl. Catal. A, 1992, 89, p. 1-30). Other catalysts reported in the patent literature include bismuth, lead, or antimony oxides (U.S. Pat. Nos. 3,644,550 and 3,830,866), chromium treated alumina (U.S. Pat. Nos. 3,836,603 and 6,600,081 B2), rhenium treated alumina (U.S. Pat. No. 4,229,320) and platinum treated zeolites (WO 2005/065393 A2).
Alternatively, low molecular weight (C2-C5) alkanes and alkenes can be treated with acidic catalysts to produce higher molecular weight (C8-C20) alkanes and alkenes. Mixtures of these alkanes and alkenes are then blended at the refinery to provide gasoline, jet, and diesel fuel. In particular, these “alkylates” can be blended with gasoline to reduce vapor pressure and raise octane value. However, unlike gasoline and diesel, jet fuel specifications cannot tolerate high quantities of olefins. To produce useful hydrocarbons suitable for jet fuel, the olefins must be reduced to saturated hydrocarbons using an additional hydrogenation step. In general, in petroleum refineries small alkanes and alkenes have little value and are processed as described above to form higher molecular weight hydrocarbons that can be blended into the higher value hydrocarbons that constitute the majority of a barrel of crude oil.
The compositions and processes of the present invention provide improved, renewable biofuels with costs and performance properties comparable to, or superior to existing biofuels and petroleum-derived fuels (e.g., jet fuels). In addition, the process of the present invention provides an integrated and simple method for producing saturated C8-C24 aliphatic hydrocarbons and aromatics from renewable alcohols (with low levels of olefins) derived from biomass. In one embodiment, the process of the present invention provides an on-specification fuel (e.g., gasoline, diesel, or jet fuel) which is completely comprised of renewable hydrocarbons.